Unfortunately, I don't believe this to be possible due to the fact that for the effects to combine,
Volrath would have to copy the other creature while the effect of the first is occurring. In this situation both effects are "dies" triggers, meaning that
Volrath would have to somehow be able to activate his ability while not on the battlefield, which I believe to be impossible (someone correct me if I'm wrong here).
This would work if you used a card like
endling instead of the ghoul because the undying effect would remain on
Volrath when you then changed him to become the hydra and sacrificed him to get the "dies" effect.
Hope this helps